How do transformers get their KVA values

Principles of transformers in parallel

introduction

To supply a load above the rating limit of If a transformer is present, two or more transformers can be connected in parallel to the existing transformer. The transformers are connected in parallel when the load on a transformer exceeds the capacity.

Principles of transformers connected in parallel (part 1)

The reliability is increased in parallel operation than if a larger unit is available.

The cost of maintaining the spare parts is lower when two transformers are connected in parallel. It is usually economical to install another transformer in parallel rather than replacing the existing transformer with a single larger unit.

The cost of a replacement device in the case of two parallel transformers (with the same power) is also lower than that of a large transformer. In addition, it is preferable to have a parallel transformer for reliability.

With this At least half of the load can be put out of service with a transformer.


Requirement for parallel operation of the transformer

To connect transformers in parallel, the primary windings of the transformers are connected to source rails and secondary windings are connected to the load rails.

Various conditions that must be met for the successful parallel operation of transformers:
  1. Same voltage and speed (both primary and secondary voltage are the same)
  2. Same percent impedance and X / R ratio
  3. Identical position of the multiple switch
  4. Same KVA ratings
  5. Same phase angle shift (vector group is the same)
  6. Same frequency weighting
  7. Same polarity
  8. Same phase sequence

Some of these conditions are practical and some are mandatory.

The favorable conditionsare: Same voltage ratio and turns ratio, same percent impedance, same KVA rating, same position of the tap changer.

The binding conditions Conditions are: same phase angle shift, same polarity, same phase sequence and same frequency. If the favorable conditions are not met, parallel operation is possible, but not optimal.


1. The same tension ratio and turning ratio (at each tap)

If the transformers are connected in parallel with slightly different voltage ratios, then due to the inequality of the induced EMF in the secondary windings, a circulating current flows in the loop formed by the secondary windings in the no-load state, which can be much larger than the normal no-load current.

The current will be quite high as the leakage current, the impedance will be low. When the secondary windings are loaded, this circulating current tends to load the two transformers unevenly and it may not be possible to take the full load from this group of two transformers in parallel (one of the transformers may become overloaded).

When two transformers with different voltage ratios are connected in parallel to the same primary supply voltage, there is a difference in secondary voltages.

Well, if the secondary of these transformers are connected to the same bus, there will be between the secondary and thus also between the primary ones2R loss.

Primary and secondary debt valuations should be identical. In other words, the transformers should have the same turns ratio; H. The transformation ratio.


2. Same percent impedance and X / R ratio

When two transformers are connected in parallel similar impedances per unit They mostly share the burden with their KVA ratings. Here the load is mostly the same as it is possible to have two transformers with the same impedances per unit, but different X / R ratios. In this case, the line current is less than the sum of the transformer currents and the combined capacitance is reduced accordingly.

A difference in the ratio of the reactance value of the resistance value of the unit impedance per unit results in a different phase angle of the currents carried by the two parallel transformers; One transformer operates at a higher power factor and the other operates at a lower power factor than the combined output. Therefore, the actual power from the transformers is not split proportionally.

The current shared by two transformers running in parallel should be proportional to their MVA values.</ p>

The current carried by these transformers is inversely proportional to their internal impedance.

From the above two statements one can say that the impedance of transformers running in parallel is inversely proportional to their MVA values. In other words, the percent impedance or the impedance values ​​per unit should be the same for all transformers running in parallel.

When connecting single-phase transformers to three-phase banks, correct impedance matching becomes even more critical. In addition to the three rules for parallel operation, it is also advisable to adjust the X / R ratios of the three series impedances in order to keep the three-phase output voltages in balance.

If single-phase transformers with the same KVA values ​​are connected in a Y-∆ bank, impedance mismatches can lead to considerable load imbalance between the transformers

Let's look at the following cases: impedance, ratio and KVA.

If single-phase transformers are connected in a Y-Y bank with isolated neutral, the magnetizing impedance should also be the same on an ohmic basis.

Otherwise the transformer has the greatest magnetizing impedance has a highest percentage of the excitation voltage, increasing the core losses of this transformer and possibly driving its core into saturation.


Case 1: Same impedance, ratios and same kVA

The standard method for connecting transformers in parallel should have the same turns ratios, percent impedances and kVA values. The parallel connection of transformers with the same parameters leads to an even load distribution and no circulating currents in the transformer windings.

example Parallel connection of two transformers with an impedance of 5,000 kVA (5.75%), each with the same winding ratio, to a load of 4,000 kVA.

  • Load on transformers-1 = KVA1 = [(KVA1 /% Z) / ((KVA1 /% Z1) + (KVA2 /% Z2))] X KVAl
  • kVA1 = 348 / (348 + 348) × 4000 kVA = 2000 kVA.
  • Load on transformers-2 = KVA1 = [(KVA2 /% Z) / ((KVA1 /% Z1) + (KVA2 /% Z2))] X KVAl
  • kVA2 = 348 / (348 + 348) × 4000 kVA = 2000 kVA
  • Therefore KVA1 = KVA2 = 2000KVA

Case 2: Same impedances, ratios and different kVA

This parameter is not common for new installations, sometimes two transformers with different kVAs and the same percentage impedances are connected to a common bus. In this situation, the current sharing means that each transformer carries its nominal load. There are no circulating currents because the voltages (winding ratios) are the same.

example Parallel connection of 3000 kVA and 1000 kVA transformers, each with 5.75% impedance and the same turns ratio, to a common 4000 kVA load

  • Load on transformer-1 = kVA1 = 522 / (522 + 174) x 4000 = 3000 kVA
  • Load on transformer-1 = kVA2 = 174 / (522 + 174) x 4000 = 1000 kVA

From the above calculation you can see the different kVA ratings for transformers connected to a common load; this current distribution has the effect that each transformer is only loaded with its kVA nominal value. The key here is that the percent impedance is the same.


Case 3: Unequal impedance, but the same ratios and kVA

Most of the time, this parameter was used to improve system performance by connecting existing transformers with the same kVA rating but different percentage impedances in parallel.

This is often the case when budget constraints limit the purchase of a new transformer with the same parameters.

We need to understand that the current divides in inverse proportion to the impedances, and a larger current flows through the smaller impedance. Thus, the transformer with lower impedance can be overloaded under heavy load, while the other transformer with lower impedance is lightly loaded.

exampleTwo 2000 kVA transformers in parallel, one with 5.75% impedance and the other with 4% impedance, each with the same turns ratio, connected to a common load of 3500 kVA.

  • Load on Transformer-1= kVA1 = 348 / (348 + 500) * 3500 = 1436 kVA
  • Load on Transformer-2 =kVA2 = 500 / (348 + 500) × 3500 = 2064 kVA

You can see this because transformer percent impedances don't match, they can't be loaded to their combined kVA values. The load sharing between the transformers is not the same. With a combined nominal load of kVA, the 4% impedance converter is overloaded by 3.2%, while the 5.75% impedance converter is loaded with 72%.


Case 4: Unequal impedance and KVA equality

This is especially true for transformers, which are rarely usedindustrial and commercial equipment connected to a common bus with different kVA and unequal percentage impedances. However, there may be a situation where two single ended substations can be connected to each other via buses or cables to provide better voltage support when starting a large load.

If the percent impedance and kVA values ​​differ, care should be taken when charging these transformers.

example Two transformers in parallel with one 3000 kVA (kVA1) with 5.75% impedance and the other 1000 kVA (kVA2) with 4% impedance, each with the same winding ratios, which are connected to a common load of 3500 kVA.

  • Load on Transformer-1 =kVA1 = 522 / (522 + 250) × 3500 = 2366 kVA
  • Load on Transformer-2 =kVA2 = 250 / (522 + 250) × 3500 = 1134 kVA

Since the percent impedance in the 1000 kVA transformer is lower, it is overloaded with less than the combined nominal load.


Case 5: Same impedance and different KVA values

Small voltage differences cause a large amount of current to circulate. It is important to point out that transformers connected in parallel must always be connected to the same tap. The circulating current is completely independent of the load and the load distribution. When the transformers are fully loaded, circulating currents cause considerable overheating.

The point that should be remember that there are no circulation currents in the pipe. They cannot be measured if there are monitoring devices before or after the common connection points.

example Two 2000 kVA transformers connected in parallel, each with 5.75% impedance, the same X / R ratio (8), transformer 1 with a tap set to 2.5% of the nominal value and transformer 2 with a nominal tap. What is the percentage circulating current (% IC)

  • % Z1 = 5.75, so% R '=% Z1 / ([(X / R) 2 + 1)] = 5.75 / ((8) 2 + 1) = 0.713
  • % R1 =% R2 = 0.713
  • % X1 =% Rx (X / R) =% X1 =% X2 = 0.713 x 8 = 5.7
  • Let% e = difference in the voltage ratio, expressed as a percentage of the normal value and k = kVA1 / kVA2
  • Circulation current% IC =% eX100 / (% R1 + k% R2) 2 + (% Z1 + k% Z2) 2
  • % IC = 2.5 × 100 / √ (0.713 + (2000/2000) × 0.713) 2 + (5.7 + (2000/2000) × 5.0)
  • % IC = 250 / 11.7 = 21.7

The circulating flow is 21.7% of full load current.


Case 6: Unequal impedance, KVA and different ratios

This type of parameter would be unlikely to train. If the ratios and impedance differ, the circulating current (because of the unequal ratio) should be combined with the load current portion of each transformer to get the actual total current in each unit.

With a power factor of 1 10% circulation current (due to unequal winding ratios) only leads to half a percent of the total current. At lower power factors, the circulating current changes dramatically.

exampleTwo transformers connected in parallel, 2000 kVA1 with an impedance of 5.75%, an X / R ratio of 8, 1000 kVA2 with an impedance of 4%, an X / R ratio of 5, 2000 kVA1, where tap 2, 5% of the nominal value and 1000 kVA2 of the nominal value is balanced.

  • % Z1 = 5.75, so% R '=% Z1 / ([(X / R) 2 + 1)] = 5.75 / ((8) 2 + 1) = 0.713
  • % X1 =% Rx (X / R) = 0.713 x 8 = 5.7
  • % Z2 = 4, so% R2 =% Z2 / [(X / R) 2 + 1)] = 4 / ((5) 2 + 1) = 0.784
  • % X2 =% Rx (X / R) = 0.784 × 5 = 3.92
  • Let% e = difference in the voltage ratio, expressed as a percentage of the normal value and k = kVA1 / kVA2
  • Circulation current% IC =% eX100 / (% R1 + k% R2) 2 + (% Z1 + k% Z2) 2
  • % IC = 2.5 × 100 / √ (0.713 + (2000/2000) × 0.713) 2 + (5.7 + (2000/2000) × 5.0)
  • % IC = 250 / 13.73 = 18.21.

The circulating flow is 18.21% of full load current.


3. Same polarity

The polarity of the transformer means the instantaneous direction of the induced emf in the secondary area. If the instantaneous directions of the induced secondary emf in two transformers are opposite to each other when the same input power is supplied to the two transformers, the transformers are said to have opposite polarity.

The transformers should be properly connected with regard to their polarity. If they are connected with wrong polarities, the two EMFs induced in the secondary windings connected in parallel act together in the local secondary circuit and create a short circuit.

The polarity of all transformers running in parallel should be the same, otherwise large circulating currents will flow in the transformer, but no load will be fed from these transformers.

If the instantaneous directions of the induced secondary emf are the same in two transformers, when the same input power is supplied to the two transformers, the transformers are said to have the same polarity.


4. Same phase sequence

The phase sequence of the mains voltages of the two transformers must be identical for the parallel operation of three-phase transformers. If the phase sequence is incorrect, each phase pair is short-circuited in each cycle.

This condition must be strictly adhered to when transformers are operated in parallel.


5.Equal phase angle shift (relative relative phase shift between the secondary voltages)

The transformer windings can be arranged in a number of ways that produce different magnitudes and phase shifts in the secondary voltage. All transformer connections can be divided into different vector groups.

Group 1: Zero phase shift (Yy0, Dd0, Dz0)
Group 2: 180 ° phase shift (Yy6, Dd6, Dz6)
Group 3: -30 ° phase shift (Yd1, Dy1, Yz1)
Group 4: + 30 ° phase shift (Yd11, Dy11, Yz11)

In order to have a relative phase shift of zero in the secondary-side mains voltages, the transformers belonging to the same group can be connected in parallel. For example, two transformers with Yd1 and Dy1 connections can be connected in parallel.

The transformers of groups 1 and 2 can only be parallel with transformers of their own group. However, the transformers of groups 3 and 4 can be connected in parallel by reversing the phase sequence of one of them. For example, a transformer with a Yd1 1 connection (group 4) can be connected in parallel with the one with a Dy1 connection (group 3) by reversing the phase sequence of the primary and secondary connections of the Dy1 transformer.

We can only be parallel Dy1 and Dy11 by crossing two incoming phases and the same two output phases at one of the transformers. So if we have a DY11 transformer we can cross the B&C phases on the primary and secondary phases to change the +30 degree phase shift to a -30 degree shift that is parallel to the Dy1, assuming all other points above are satisfied.


6. Same KVA ratings

If two or more transformers are connected in parallel, the load distribution in% is based on their nominal value. When everyone has the same rating, they share the same burden

Transformers with unequal kVA values ​​share aThe load is charged practically (but not exactly) proportionally to their rated powers, provided that the voltage ratios are identical and the percent impedances (for their own kVA rated power) are identical or in these cases almost the same Usually 90% of the total of the two ratings is available.

It is recommended that transformers whose kVA rated values ​​differ by more than 2: 1 are not operated in parallel over the long term.

Transformers with different kva values ​​can work in parallel with a load sharing, so that each transformer carries its proportional share of the total load.To achieve an exact load sharing, the transformers must be wound with the same turns ratio and the impedance percentage of all transformers be the same if every percentage is specified on the basis of the respective transformer. It is also necessary that the resistance to reactant ratio be the same in all transformers.

For satisfactory operation, the circulating current should probably not be more than ten percent of the rated current of the smaller unit for any combination of ratios and impedance.


7. Identical tap changer and its operation

The only important point to remember is that the on-load tap changers for all three transformers must be in the same position and check that the secondary voltages are the same.

If the voltage tap has to be changed, all three taps must be changed. Changeover switches should be operated in the same way for all transformers. The OL settings of the SF6 should also be identical. When the substation is operating at full load, the tripping of one transformer can result in cascade tripping of all three transformers.

In transformers, the output voltage can be controlled either with the step switch "From the circuit" (manual step change) or "By loading - step switch OLTC" (automatic change).

In the transformer with OLTC it is a closed system with the following components:

1.AVR (automatic voltage regulator) - an electronically programmable device). With this AVR we can adjust the output voltage of the transformers. The output voltage of the transformer is fed into the AVR via the LT panel. The AVR compares the SET voltage and the output voltage and, if present, sends the error signals via the RTCC panel to the OLTC to change the tap. This AVR is mounted in the RTCC.

2. RTCC (Remote Tap Change Cubicle) - This is a panel consisting of the AVR, display for step position, voltage and LEDs for relays for raising and lowering the steps, selector switch for automatic manual selection ... In AUTO MODE, the voltage is controlled by the AVR. In manual mode, the operator can increase or decrease the voltage by manually changing the taps using the push button in the RTCC.

3. OLTC is mounted on the transformer - It consists of a motor controlled by the RTCC that changes the taps in the transformers.

Both transformers should have the same voltage at all taps and if you operate transformers in parallel it should have the same tap position. If we have OLTC with RTCC panels, one RTCC should work as a master and another should work as a follower to maintain the same tap positions of the transformer.

However, a circulating current can flow between the two tanks if the impedances of the two transformers are different or if the steps of the on-load tap-changer (OLTC) temporarily do not match due to the mechanical delay. The circulating current can cause the protection relays to malfunction.

</ p>
References
  • Say M.G. The performance and design of AC machines.
  • Application Guide, Transformer Loading, Nashville, TN, USA.
  • Toro, V.D. Principles of electrical engineering.
  • Stevenson, W.D. Elements of the energy system analysis.
  • MIT Press, Magnetic Circuits and Transformers, John Wiley and Sons.